The present invention relates to the field of automatic transmissions, and, more particularly, relates to the field of automatic transmissions with torque converters which are equipped with lock up clutches.
Various automatic transmissions for automotive vehicles are known in various forms. Such an automatic transmission conventionally includes a gear transmission mechanism which provides a plurality of speed stages and which is set to its various speed stages by selective supply of actuating hydraulic fluid pressures to various friction engaging mechanisms comprised within the gear transmission mechanism such as multi plate clutches and multi plate brakes, said selective supply of actuating fluid pressures being provided from a hydraulic fluid pressure control system, of which many forms are well known. Such a hydraulic fluid pressure control system typically requires a supply of line pressure for operation, and typically receives this line pressure from a hydraulic fluid pump by way of a line pressure regulation valve which modifies the output fluid pressure produced by the pump by releasing a part of it through a release port. And it is typical for such a hydraulic fluid pump to be rotated at a rotational speed proportional to the rotational speed of the power input shaft of the transmission, i.e. at a rotational speed at least approximately proportionally related to the rotational speed of the crankshaft of the internal combustion engine which powers the vehicle.
Further, such an automatic transmission conventionally includes a fluid torque converter, which provides a fluid coupling between an engine of the vehicle and the gear transmission mechanism, thus eliminating the need for any clutch system for the vehicle, and allowing for the vehicle to be stationary while the engine is turning at a low rotational speed at or close to the idling speed, as well as providing torque multiplication by fluid flow, in a per se well known way, when the vehicle is being accelerated at relatively low speed and relatively low engine rotational speed. Many such torque converters are of course presently well known. Generally, such a torque converter comprises: a housing of a generally toroidal shape, on the inside of which there are formed a series of vanes which constitute a pump impeller, and fixed to a power input shaft; a pump turbine member mounted within the housing as fixed to a power output shaft; and a stator member mounted within the housing via a one way brake on a fixed member. The housing of such a torque converter is kept filled with hydraulic fluid, which is pumped thereinto and is also drained therefrom, and in a per se well known way the pump impeller, the stator member, and the turbine member cooperate, when the housing of the torque converter is thus filled with hydraulic fluid, to define a toroidal hydraulic fluid flow circulation system, which is adapted to transfer torque in a conventional fashion between the pump impeller and the turbine member.
This supply of hydraulic fluid for filling the torque converter is typically provided to the inside of the housing thereof along or beside the central rotational axis thereof via a hole in one of the shafts passing therealong, or between two concentric ones of such shafts, and the draining of hydraulic fluid from the torque converter is also typically performed in a similar manner. The supply of hydraulic fluid is provided, generally in the prior art, from a torque converter pressure regulation valve, which, in a fashion similar to the line pressure regulation valve mentioned above, modifies the release fluid pressure discharged from the release port of the line pressure regulation valve by releasing a part of it back to a hydraulic fluid reservoir, and which supplies a torque converter pressure to the torque converter. Further, it has become more and more common nowadays for a torque converter to be provided with a lock up clutch, which is a mechanical clutch which when actuated mechanically couples together the pump impeller and the pump turbine member of the torque converter with regard to their rotation, so that the above mentioned hydraulic torque transmission between the pump impeller and the pump turbine no longer occurs. Such a lock up clutch is engaged, typically, when the vehicle incorporating the torque converter is being driven at high road speed with the gear transmission mechanism in a high gear speed stage, and with the internal combustion engine of the vehicle operating at fairly high rotational speed, in which circumstances the hydraulic torque conversion function of the torque converter is not in fact particularly required; and the provision of such a lock up clutch is effective for increasing fuel economy of the vehicle, especially when running on the open road such as an expressway. It is well known and conventional for such a lock up clutch to be engaged or disengaged according to the directions of supply and draining of the torque converter pressure to and from the interior of the housing of the torque converter. In other words, when the torque converter pressure is being supplied to one aperture which leads to the interior of the torque converter housing, and is being drained from another aperture, then the lock up clutch is engaged; and when the torque converter pressure is being supplied to said other aperture, and is being drained from said one aperture, then the lock up clutch is disengaged. Thus the supply of torque converter hydraulic fluid pressure to the torque converter from the torque converter pressure regulation valve is used for two purposes: to fill the torque converter with hydraulic fluid, and to selectively engage and disengage the lock up clutch, according to the direction of said supply.
Meanwhile, the above mentioned gear transmission mechanism of the automatic transmission of course requires lubrication, which typically is performed by supply of the same hydraulic fluid as is used for the hydraulic operation of the torque converter and of the hydraulic fluid pressure control system, and this supply of lubricating hydraulic fluid to the gear transmission mechanism is conventionally made by diverting part of the supply of the torque converter hydraulic fluid pressure which is being supplied to the torque converter and to the lock up clutch from the torque converter pressure regulation valve.
Further, heat tends to be generated within the torque converter when the torque converter is performing its function of fluid torque conversion, i.e. when the lock up clutch is not engaged, in quite a large amount, due to the churning of hydraulic fluid therein between the vanes formed on the pump impeller, the stator, and the pump turbine member, and due to frictional losses in the torque converter, and accordingly it is usual for a hydraulic fluid cooler to be provided, through which the flow of hydraulic fluid which has passed through the torque converter is conducted, in order to keep the rise in temperature of the hydraulic fluid within the transmission to within a reasonable and acceptable level.
However, in the prior art, difficulties have arisen with regard to the proper disengagement of the lock up clutch, when as described above the torque converter pressure is being supplied to the torque converter and to the lock up clutch from the torque converter pressure regulation valve which modifies the fluid pressure discharged from the line pressure regulation valve. In detail, when the internal combustion engine of the vehicle incorporating the transmission is operating at fairly high rotational speed, and accordingly the hydraulic fluid pump of the transmission, as described above, is rotating at a fairly high rotational speed and accordingly is providing fairly high delivery rate of pressurized hydraulic fluid, then no problem tends to arise; but, when the internal combustion engine of the vehicle incorporating the transmission is operating at a low rotational speed, and accordingly the hydraulic fluid pump, as described above, is rotating at a fairly low rotational speed and accordingly is providing a fairly low delivery rate of pressurized hydraulic fluid, then the release rate from the line pressure regulation valve may prove inadequate for providing a source of hydraulic fluid pressure to be regulated by the torque converter pressure regulation valve for performing the functions of providing a supply of lubricating hydraulic fluid to the gear transmission mechanism and also providing a flow of hydraulic fluid through the torque converter in the appropriate direction for disengaging the lock up clutch and through the hydraulic fluid cooler for cooling the hydraulic fluid contained within the transmission.
In these circumstances, typically no problem arises with regard to lubrication of the gear transmission mechanism, because when the internal combustion engine of the vehicle incorporating the transmission is operating at a low rotational speed then in general the moving parts of the gear transmission are also operating at low rotational speeds and under low force loadings, and hence these moving parts do not require supply of a particularly large amount of lubricating hydraulic fluid. Similarly, no problem typically arises in the above defined circumstances with regard to cooling of the hydraulic fluid within the transmission, because when the internal combustion engine of the vehicle incorporating the transmission is operating at a low rotational speed then in general no very great amount of heat is being generated within the torque converter by the churning of hydraulic fluid therein, even though the lock up clutch is disengaged. However, a problem has arisen in the prior art with regard to the proper disengagement of the lock up clutch, and when as has often been the case the supply of flow of hydraulic fluid through the torque converter, performed from the torque converter pressure regulation valve, has been insufficient fully to disengage the lock up clutch, dragging, chattering, and improper release of the lock up clutch have tended to occur, which have deteriorated the drivability of the vehicle incorporating the transmission, as well as causing poor durability of the lock up clutch.